Alarm apparatus

ABSTRACT

An alarm apparatus comprises detector means for detecting the number of rotation of a rotating body as an electrical intermittent frequency, a digital-analog converter for converting the intermittent frequency from the detector means into a DC voltage, a voltage comparator for discriminating the amplitude of an output voltage from the digital-analog converter, an alarm unit controlled by an output from the voltage comparator and feedback means for feeding back the output from the voltage comparator to the input side of the digital-analog converter. The voltage comparator serves as a low-frequency oscillator in cooperation with the feedback means thereby to sound an intermittent alarm.

United States Patent 1191 Okada et al. 5] Apr. 8, 1975 [5 1 ALARM APPARATUS 3.204.183 8/1965 Hasenzahl 340/384 E 1 1751 Masashi Okada, Kariya; Yoshihiko 38???? 131133? 235323851111: 111111336337? Konfio, fw Tadao t 3,755,688 8/1973 Hohler 340/256 x Karlya; Yuklo Muramatsu, N1shio, of Japan Primary E.\'amir1erDavid L. Trafton [73] Assignee: Nippondenso Co., Ltd., Kariya-shi, Attorney, Agent, or F irn1-Cushmar1, Darby &

Aichi-ken, Japan Cushman [22] Filed: Oct. 5, 1973 211 App]. No: 403,862 [57] ABSTRACT An alarm apparatus comprises detector means for de- [30] Foreign Application Priority Data tectmg the number of rotat1on of a rotatmg body as an a 7 electncal mtermtttent frequency, a dlgltal-analog con- Oct. 9, I97- Japan 47-l()l4 2 verter for converting the intermittent frequency from Oct. 16, l97- Japan 47-103368 the detector means into a DC voltage a voltage parator for discriminating the amplitude of an output [52] US. Cl. 340/271, 331/64, 331/65, voltage from the digital analog converter an alarm 340/263; 340/384 E unit controlled by an output from the voltage compar- 15 1 11 x131. G08b 21/00; G08b 3/10 mot and feedback means for feeding back the Output [58] held 61 Search 340/271, 263, 384 E, from the voltage comparator to the input Side of the 331/64 digital-analog converter. The voltage comparator s serves as a low-frequency oscillator in cooperation 6] References C'ted with the feedback means thereby to sound an intermit- UNITED STATES PATENTS tent alarm 2,447,156 8/l948 Brittain 340/384 E 2.988.708 6/1961 Schmidt 340/384 E 7 Clams 11 Drawmg Flgures FEEDBACK MEANS I 2 3 4 K R K DETECTOR D-A VOLTAGE ALARM CONVERTER COMPARATOR UNIT tam/.004

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t TIME ALARM APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alarm apparatus, and more particularly to an alarm apparatus which supplies an intermittent signal of low repetition frequency to an alarm unit when an input frequency in response to a detectable quantity such as the number of rotation exceeds a predetermined frequency value.

Preferably, the present invention is applicable to an alarm apparatus in which when wheel speed or the number of engine rotation. for example, exceeds a limit value, an intermittent output signal of low frequency is generated which in turn sound an alarm unit (for example a bell).

2. Description of the Prior Art ART Conventionally, there has been proposed an alarm apparatus in which after an intermittent signal responsive to the number of rotation ofa rotating body undergoes a digital-analog conversion (frequency to voltage conversion), a DC output voltage from the digitalanalog converter is compared with a reference DC voltage and an output of the comparator enables an oscillator circuit to energize an alarm unit intermittently. However, since the alarm apparatus of prior art mentioned above requires a particular oscillator circuit, its circuit construction is complicated with the increase in the number of parts. Further, in the case that where a single input frequency is detected by a plurality of de' tector channels, a plurality of voltage comparators each including a transistor or transistors of low input impedance are connected in parallel with the output of the digital-analog converter so that when one of the voltage comparators is operated, its operation influences the output voltage of the digital-analog converter thereby causing an interference between the detector channels. Accordingly, it was usual to provide a digitalanalog converter for respective detector channels.

SUMMARY OF THE INVENTION The present invention intends to obviate the aforementioned disadvantages. A principal object of the present invention is toprovide an improved and simple alarm apparatus capable of sounding an intermittent alarm signal which comprises a low-frequency oscillator circuit established by the feedback from the digitalanalog converter to the voltage comparator and dispenses with a particular oscillator circuit of the prior art.

Another object of the present invention is to provide a multi-channel frequency detecting apparatus operative stably and constructed of a minimized number of circuit elements which comprises, between the digitalanalog converter for converting an input frequency of the detector means into a DC voltage and a plurality of voltage comparators for discriminating the amplitude of an output voltage of the digital-analog converter, signal transfer circuit including a high input impedance active element (for example an emitter follower transistor), whereby a single digital-analog converter permits a smooth operation of the plurality of detector channels without causing interference therebetween.

According to one aspect of the present invention, an alarm apparatuscbmprises a combination of a digitalanalog converter and voltage comparator,'and a lowfrequency oscillator circuit established by feeding back an output of the voltage comparator to the input side of the digital-analog converter so that the frequency detector serves as an oscillator. Thus, it is possible to sound an intermittent alarm with a simple circuit arrangement, dispensing with a particular oscillator circuit of the prior art when a quantity to be detected reaches a predetermined value.

According to another aspect of the present invention, a multi-channel frequency detecting apparatus comprises a signal transfer circuit including a high input impedance active element so as to effect a stable and sure detection ofa single input frequency by the plurality of detector channels without causing any interference between the channels. Thus, it is possible to ensure a stable operation of the apparatus with a simple circuit arrangement including a single digital-analog converter.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantages of the invention will be better understood from the following de tailed description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram of an alarm apparatus of the invention;

FIG. 2 is a circuit diagram of the first embodiment of the invention;

FIGS. 3a and 3b are waveform diagrams used to explain the operation in accordance with the invention;

FIG. 4 is a circuit diagram of the second embodiment of the invention;

FIG. 5 is a circuit diagram of the third embodiment of the invention;

FIG. 6 is a circuit diagram of the fourth embodiment of the invention;

FIG. 7 is a circuit diagram of the fifth embodiment of the invention;

FIG. 8 is a circuit diagram of the sixth embodiment of the invention; and

FIGS. 9a and 9b are waveform diagrams used to explain the operation of the alarm apparatus shown in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An alarm apparatus generally comprises as shown in FIG. 1 detector means I for detecting a detectable quantity such as the number of rotations of a rotating body as an electric intermittent frequency,a digitalanalog converter 2 for converting the intermittent frequency from the detector means 1 into a DC voltage, a voltage comparator 3 for discriminating the amplitude of an output voltage from the digital-analog converter 2, an alarm unit 4 controlled by an output from the voltage comparator 3 and feedback means 5 for feeding back an output from the voltage comparator 3 to the input side of the digital-analog converter 2, wherein the voltage comparator 3 serves as a lowfrequency oscillator in cooperation with the feedback means 5.

Referring now to FIG. 2, the first embodiment of the invention will be described which may be reduced to such a practice that the wheel speed of an automobile is detected to energize an alarm unit such as a bell intermittently for sounding an alarm when the wheel speed exceeds a predetermined value. In the figure, nume ral 11' designates a permanent magnet rotatable at the number of rotation in response to the wheel speed and driven through a speed meter cable (not shown), 12 a reed switch intermittently switched by the permanent magnet 11, and 13 a terminal at which an intermittent signal 14 of a frequency responsive to the wheel speed appears. Numeral 21 represents a resistor, 22 a differentiating capacitor, 23 a charging diode, 24 a discharging diode and 25 an integrating capacitor. Numeral 2 generally designates the digital-analog converter where a DC voltage in response to an intermittent frequency of the intermittent signal 14 is developed across the integrating capacitor 25. Numerals 31 and 32 designate base bias resistors, 33 and 34 transistors with a common emitter resistor 35 constituting a Schmidt circuit, 36 and 37 collector load resistors for transistors, 38 and 39 base bias resistors, 391 a load driving transistor, 392 a diode for absorbing a surge voltage, and 393 an output terminal of the voltage comparator generally designated at numeral 3. Numeral 51 represents a feedback resistor constituting the feedback meansS. Numeral 6 represents a power terminal to be connected to a positive electrode of a battery (not shown). An alarm unit 4 is a bell.

In the operation, the reed switch 12 of detector means 1 is switched intermittently in response to the wheel speed of automobile so that the frequency of intermittent signal 14 appearing at the terminal 13 varies dependent on the wheel speed. During the period for low wheel speed, since the number of times for charging to the integrating capacitor 25, through the differentiating capacitor 22 is small, a stored voltage across the integrating capacitor 25 is low so that, in the voltage comparator 3, the transistor 33 is off, transistor 34 on and transistor 391 off, thereby deenergizing the alarm unit 4. As the vehicle speed gradually increases until it reaches a predetermined value, a voltage V across the integrating capacitor 25 of digital-analog converter 2 increases to the voltage V as shown in FIG. 3a, which enables the transistor 33 of voltage comparator 3 to turn on with the result that the transistor 34 becomes off and the transistor 391 on, thereby energizing the alarm unit 4 for sounding an alarm. At the same time, an output of the voltage comparator 3 is fedback to the input side of digital-analog converter 2 through the feedback resistor 51. In other words, one end of the feedback resistor 51 falls to ground potential through the collector and emitter of transistor 391 and accordingly the differentiating capacitor 22 is prevented from charging and discharging even when the reed switch 12 is operated, thus preventing the integrating capacitor 25 from being charged. This causes the integrating capacitor 25 to act merely to discharge through the resistors 31 and 32, the base and emitter of transistor 33 and the emitter resistor 35, and accordingly the voltage V across the integrating capacitor 25 decreases in accordance with a time constant which is determined by circuit elements included in the aforementioned discharge circuit. When the decrease in voltage V across the integrating capacitor 25 reaches the value V as shown in FIG. 3a, which is determined by a hysteresis width of the voltage comparator 3, the voltage comparator 3 is inverted with the transistor 33 off, transistor 34 on and transistor 391 off thereby to break off power to the alarm unit 4 with the stopping of the sounding of an alarm. Simultaneously, since the transistor 391 is off, one end of the feedback resistor 51 reverts, to power source potential so that the integrating capacitor 25 is recharged through the differentiating capacitor 22 with a gradual increase in the voltage V across the integrating capacitor 25. However, the voltage comparator 3 is maintained at the abovementioned state below V of the voltage V across the integrating capacitor 25 as shown in FIG. 3a Then, when the voltage V reaches to V,, the voltage comparator 3 is inverted. In like manner, the voltage comparator 3 repeats its inversion dependent on a period of time delay required for charging and discharging the integrating capacitor 25 and serves as a low-frequency oscillator circuit. To the alarm unit 4 supplied is an intermittent voltage V as shown in FIG. 3b for intermittently sounding an alarm.

FIG. 4 shows the second embodiment of the invention wherein a resistor 26 is added to the digital-analog converter 2. In this embodiment. since a voltage divided by the resistor 26 and the feedback resistor 51 is applied to the integrating capacitor 25 through the differentiating capacitor 22 when the transistor 391 of voltage comparator 3 is on, the integrating capacitor 25 is slightly charged. Consequently, if a resistance of the resistor 26 is set such that the voltage V of integrating capacitor 25 does not decrease below the voltage V (recovering voltage of the voltage comparator) shown in FIG. 3a for a high wheel speed excessive to V corresponding to the voltage V shown in FIG. 3a, the alarm unit 4 continuously sounds for a wheel speed larger than V,. In other words, in accordance with the second embodiment the alarm unit 4 intermittently sounds for a wheel speed between V corresponding to the voltage V and V, corresponding to the voltage V, shown in FIG. 30 whereas it continuously sounds for a wheel speed larger than V Thus, the alarm unit can sound an alarm in two steps.

FIG. 5 shows the third embodiment of the invention wherein an integrating capacitor 25' and a resistor 27 for integration included in the digital-analog converter 2 are connected with the source terminal 6 (positive electrode of power source). With this circuit arrangement, since a voltage applied to the base of transistor 33 of the preceding stage in voltage comparator 3 is decreased as the vehicle speed increases, the voltage comparator 3 operates in an inversive manner to the first and second embodiments, thereby dispensing with the load driving transistor 391.

FIG. 6 shows the fourth embodiment wherein the digital-analog converter 2 includes a transistor 28 of base grounding. By constantly charging the integrating capacitor 25 with a constant current irrespective of a voltage across the integrating capacitor 25, the voltage of the integrating capacitor 25 can vary linearly in relation to the wheel speed. In addition, the voltage comparator 3 includes an operation amplifier 394 which acts as a switching amplifier by connecting a positive feedback resistor 395 between its output terminal and the non-inverting input terminal.

FIG. 7 shows the fifth embodiment of the invention wherein the digital-analog converter 2 of the third embodiment shown in FIG. 5 is modified. More particularly, a transistor 29 is turned on by a charging current of the differentiating capacitor 22 and a charging current for the integrating capacitor 25' is passed through the collector and emitter of transistor 29. It was found from an experiment that the voltage characteristics of digital-analog conveter 2 were improved according to the fifth embodiment shown in FIG. 7.

While, in the foregoing embodiments, the alarm apparatus has been applied to monitor wheel speed, it is also applicable to monitoring various detectable quantities other than wheel speed, for example the number of rotations of an engine, as far as frequency is representative of an input signal. It is noted that the alarm unit may be reduced to a pilot lamp instead of a bell.

Referring now to FIG. 8, the sixth embodiment of the invention will be described which is reduced to a wheel speed detecting apparatus for automobiles. In the figure, numeral 1 generally designates detector means for detecting a detectable quantity is wheel speed of an automobile in this embodiment) as an electric intermittent frequency', which includes a permanent magnet 11 rotatable at the number of rotations responsive to the wheel speed and driven through a speed meter cable (not shown) and a reed switch 12 intermittently switched by the permanent magnet 11. Numeral 13 designates a terminal at which an intermittent signal of a frequency responsive to the wheel speed appears. Numeral 2 generally represents the digital-analog converter which includes a differentiating capacitor 22, an amplifying transistor 29, a base resistor 123, a base bias resistor 124, a reverse current preventing diode 125, an integrating capacitor 25', a collector resistor 127 for the transistor 29 and a reverse current prevention diode 128. The digital-analog converter 2 is adapted to produce a DC voltage responsive to an intermittent frequency of the intermittent signal 14 across the integrating capacitor 25. Numeral 2' generally represents a signal transfer circuit which amplifies an output voltage of the digital-analog converter 2 to sent it to two voltage comparators 3 and 3 hereinafter described. The

signal transfer circuit 2' includes a high input impedance emitter follower transistor 131, a base resistor 132 and emitter resistors 133 and 134. Numerals 3 and 3 represent separate voltage comparators each constituted by a well-known Schmidt circuit having a hystere sis effect. The voltage comparator 3 includes a variable resistor 141 for adjusting an operating point of the comparator 3, transistors 142 and 143, a common emitter resistor 144, a collector load resistor 145 for the transistor 142, and a surge voltage absorbing diode 146. The voltage comparator 3' includes a variable resistor 151 for adjusting an operating point of the comparator 3', transistors 152 and 153, a common emitter 154, a collector load resistor 155 for the transistor 152, and a surge voltage absorbing diode 156. Numeral 4 designates the alarm unit which is a load to the voltage comparator 3 and is reduced to a chime in this embodiment. Numeral 7 designates a solenoid for actuating a throttle which is a load to the voltage comparator 3. Numeral 51 designates a feedback resistor for feeding back an output from the voltage comparator 3 to the input side of digital-analog converter 2. Numeral 9 designates a battery source, 91 an ignition switch of an automobile and 92 a fuse. t

In the operation, the reed switch 12 of detector means 1 is switched intermittently in response to the wheel speed so that the frequency of intermittent signal 14 appearing at the terminal 13 varies with the wheel speed. During the period of low wheel speed, since the number of charges and discharges of the differentiating capacitor 22 is small, the number of conductions of the transistor 29 is small which is turned on by a charging current to the differentiating capacitor 22. As a result, the number of times for charging to the integrating capacitor 25' through the collector and emitter of transistor 29 is decreased to lower the voltage across the integrating capacitor 25' so that the emitter follower transistor 131 of the signal transfer circuit 3 is deeply forwardlybiased. This causes the emitter current of emitter follower transistor 131 to increase, thus turning on both the transistors 142 and 152 of the preceding stages in the voltage comparators 3 and 3'. At the same time, the transistors 143 and 153 of succeeding stages are turned off and both the alarm unit 4 and solenoid 7 for actuating a throttle are deenergized.

As the vehicle speed gradually increases, the'number of times for charging to the integrating capacitor 25' increases, resulting in the increment of voltage across the capacitor 25'. As a result, the base potential of emitter follower transistor 131 in the signal transfer circuit 2' is gradually decreased to lower the emitter current. Accordingly, when the wheel speed reaches to a first set wheel speed determined by the voltage comparator 3', for example 20 Km/hour, the transistor 152 of the preceding stage in the voltage comparator 3' is inverted to be non-conductive by the aforementioned decrease in emitter current. Thisbrings the transistor 153 of the succeeding stage to conductive state thereby to energize the solenoid 7 for the throttle. For a wheel speed above the set value (20 Km/hour in this embodiment), the solenoid 7 acts to slightly open a throttle valve by a predetermined opening even when anaccelerator pedal is released, and it is used for cleaning exhaust gases from the engine of an automobile. On the other hand, the voltage comparator 3 remains not inverted at that time so as to prevent the alarm unit from operating. As the wheel speed furthr increases and reaches to a second set value determined by the'voltage comparator 3, for example 1.00 Km/hour, the voltage across the integrating capacitor 25 of digital-analog converter reaches V, shown in FIG. 2a and an decrease in the emitter current of emitter follower transistor 131 disable the transistor 142 of the preceding stagein the voltage comparator 3, thereby enabling the transistor 143 of the succeeding stage to energize the alarm unit 4. Simultaneously, the output of the voltage comparator 3 is fedback to the input side of digital-analog converter 2 through the feedback resistor 51. That is, one end of the feedback resistor 51 falls to ground potential through the collector and emitter of transistor 143 with the result that even when the reed switch 112 is switched, the differentiating capacitor 22 is prevented from charging and discharging and the integrating capacitor 25 cannot be charged. Thus, the integrating capacitor 25' acts only to discharge through the resistor 127 and the diode 128 with the decrease in voltage across the capacitor 25 relying on a time constant determined by circuit elements of the aforementioned discharge circuit. When the voltage V across integrating capacitor decreases by the amount of the hysteresis width of the voltage comparator 3 to reach V shown in FIG. 2a, the increase in emitter current of the emitter follower transistor 131 makes the voltage comparator 3 inverted so that the transistor 142 of preceding stage becomes conductive and the transistor 143 of the succeeding stage non-conductive, thereby preventing the alarm unit 4 from sounding an alarm. Concurrently therewith, non-conduction of the transistor 143 of succeeding stage causes one end of the feedback resistor 51 to recover to the power source potential, and the integrating capacitor 25 is recharged through the differentiating capacitor 22 with a gradual increase in the voltage V across the integrating capacitor However, the voltage comparator 3 remains at the aforementioned state before the voltage V of the integrating capacitor 25' reaches V shown in FIG. 2a and it is inverted when the voltage V reaches to V In like manner, the voltage comparator 3 repeats its inversion dependent on a period of time delay required for charging and discharging the integrating capacitor 25' and serves as a low-frequency oscillator circuit. To the alarm unit 4 applied is an intermittent voltage V as shown in FIG. 9b for intermittently sounding an alarm. The intermittent sounding effectively tells a driver that the wheel speed has reached above 100 Km/hour. As shown in FIG. 9a, if the voltage V across the integrating capacitor 25 fluctuates between V and V there arises no influence on operation of the voltage comparator 3. This is because the voltage V of integrating capacitor 25' has a value V which is far smaller than V and V for actuating the voltage comparator 3.

As has been described, a wheel speed detecting apparatus of the invention having a plurality of detector channels wherein a single input frequency is detected by two voltage comparators 3 and 3, comprises the signal transfer circuit 2' including a high input inpedance emitter follower transistor 131 inserted between the digital-analog converter 2 and the two voltage comparators 3 and 3' for amplifying an output voltage of the digital-analog converter 2 so as to send it to separate two voltage comparators 3 and 3', whereby the operation of either voltage comparator 3 or 3 which may have a low input impedance does not influence the digital-analog converter 2 without causing an interference between the two voltage comparators 3 and 3'.

While, in the foregoing embodiment, a wheel speed detecting apparatus having two detector channels has been described, the present invention is applicable to multiple detector channels, that is more that three detector channels, of course. It should be noted that the signal transfer circuit 2' can be constituted by a high input impedance active element other than the emitter follower transistor 131, for example operational amplitiers.

Further, it should be understood that the digitalanalog converter 2 and voltage comparators 3 and 3' have been described in a preferred fashion in the foregoing embodiments and various modifications thereof may be possible.

What is claimed is:

1. An alarm apparatus comprising detector means for detecting a detectable quantity such as the number of rotation of a rotating body as an electric intermittent frequency, a digital-analog converter for converting the intermittent frequency from said detector means into a DC voltage, at least one voltage comparator for discriminating the amplitude of an output voltage from said digital-analog converter and producing a warning signal when the output voltage exceeds a given level, an alarm unit connected to and controlled by said warning signal from said voltage comparator and feedback means for feeding back the output from said voltage comparator to the input side of said digital-analog converter, whereby said voltage comparator serves as a low-frequency oscillator circuit in cooperation with said feedback means.

2. An alarm apparatus comprising detector means for detecting a detectable quantity such as the number of rotation of a rotating body as an electric intermittent frequency, a digital-analog converter for converting the intermittent frequency from said detector means into a DC voltage, an emitter follower transistor, the base circuit of which is connected to the output side of said digital-analog converter, a plurality of voltage comparators, each thereof connected to the emitter circuit of said emitter follower transistor, for discriminating the amplitude of an output voltage from said digital-analog converter, feedback means for feeding back an output voltage from one of the plurality of voltage comparators having the highest operating voltage to the input side of said digital-analog converter, whereby said voltage comparator having the highest operating voltage serves as a low-frequency oscillator circuit in cooperation with said feedback means and controls an alarm unit 3. An alarm apparatus according to claim 1, wherein said feedback means comprises a resistor connected between the output of said voltage comparator and the input of said digital-analog converter, and said digitalanalog converter comprises a differentiating capacitor with one end connected to the input of said digitalanalog converter and the other endto ground through a first diode and an integrating capacitor with one end connected to said differentiating capacitor through a second diode and the other end to ground.

4. An alarm apparatus according to claim 3 which further comprises a resistor connected between a power source and the input of said digital-analog converter.

5. An alarm apparatus according to claim 1, wherein said feedback means comprises a resistor connected between the output of said voltage comparator and the input of said digital-analog converter, and said digitalanalog converter comprises a differentiating capacitor with one end connected to the input of said digitalanalog converter and the other to a power source through a first diode and an integrating capacitor with one end connected to said differentiating capacitor through a second diode and the other to the power source.

6. An alarm apparatus according to claim 1, wherein said feedback means comprises a resistor connected between the output of said voltage capacitor and the input of said digital-analog converter, and said digitalanalog converter comprises a p-n-p transistor whose base is connected to a power source, emitter to the input of said digital-analog converter through a differentiating capacitor and collector to ground through an integrating capacitor.

7. An alarm apparatus according to claim 1, wherein said feedback means comprises a resistor connected between the output of said voltage comparator and the input of said digital-analog converter, and said digitalanalog converter comprises an n-p-n transistor whose base is connected to the input of said digital-analog converter through a differentiating capacitor, emitter to ground and collector to a power source through an integrating capacitor. 

1. An alarm apparatus comprising detector means for detecting a detectable quantity such as the number of rotation of a rotating body as an electric intermittent frequency, a digital-analog converter for converting the intermittent frequency from said detector means into a DC voltage, at least one voltage comparator for discriminating the amplitude of an output voltage from said digital-analog converter and producing a warning signal when the output voltage exceeds a given level, an alarm unit connected to and controlled by said warning signal from said voltage comparator and feedback means for feeding back the output from said voltage comparator to the input side of said digital-analog converter, whereby said voltage comparator serves as a lowfrequency oscillator circuit in cooperation with said feedback means.
 2. An alarm apparatus comprising detector means for detecting a detectable quantity such as the number of rotation of a rotating body as an electric intermittent frequency, a digital-analog converter for converting the intermittent frequency from said detector means into a DC voltage, an emitter follower transistor, the base circuit of which is connected to the output side of said digital-analog converter, a plurality of voltage comparators, each thereof connected to the emitter circuit of said emitter follower transistor, for discriminating the amplitude of an output voltage from said digital-analog converter, feedback means for feeding back an output voltage from one of the plurality of voltage comparators having the highest operating voltage to the input side of said digital-analog converter, whereby said voltage comparator having the highest operating voltage serves as a low-frequency oscillator circuit in cooperation with said feedback means and controls an alarm unit.
 3. An alarm apparatus according to claim 1, wherein said feedback means comprises a resistor connected between the output of said voltage comparator and the input of said digital-analog converter, and said digital-analog converter comprises a differentiating capacitor with one end connected to the input of said digital-analog converter and the other end to ground through a first diode and an integrating capacitor with one end connected to said differentiating capacitor through a second diode and the other end to ground.
 4. An alarm apparatus according to claim 3 which further comprises a resistor connected between a power source and the input of said digital-analog converter.
 5. An alarm apparatus according to claim 1, wherein said feedback means comprises a resistor connected between the output of said voltage comparator and the input of said digital-analog converter, and said digital-analog converter comprises a differentiating capacitor with one end connected to the input of said digital-analog converter and the other to a power source through a first diode and an integrating capacitor with one end connected to said differentiating capacitor through a second diode and the other to the power source.
 6. An alarm apparatus according to claim 1, wherein said feedback means comprises a resistor connected between the output of said voltage capacitor and the input of said digital-analog converter, and said digital-analog converter comprises a p-n-p transistor whose base is connected to a power source, emitter to the input of said digital-analog converter through a differentiating capacitor and collector to ground through an integrating capacitor.
 7. An alarm apparatus according to claim 1, wherein said feedback means comprises a resistor connected between the output of said voltage comparator and the input of said digital-analog converter, and said digital-analog converter comprises an n-p-n transistor whose base is connected to the input of said digital-analog converter through a differentiating capacitor, emitter to ground and collector to a power source through an integrating capacitor. 